1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to an output buffer circuit of the semiconductor device.
2. Description of the Related Art
A prior art output buffer circuit is constructed by a logic circuit for generating first and second data signals each having a voltage level between a ground level GND and a high voltage V.sub.DDL such as 3V, level shift circuits for changing the high voltage level (=V.sub.DDL) of the first and second data signals to a higher voltage level V.sub.DDH such as 5V, and an output circuit formed by a P-channel metal oxide semiconductor (MOS) transistor and an N-channel MOS transistor powered by the ground level GND and the high voltage V.sub.DDH. The P-channel MOS transistor is controlled by the level-shifted first data signal, and the N-channel MOS transistor is controlled by the level-shifted second data signal. Thus, an output signal having a voltage level between the ground level GND and the high voltage V.sub.DDH is obtained from the output circuit. This will be explained later in detail.
In the above-mentioned prior art output buffer circuit the voltage V.sub.DDH, which is larger than the maximum voltage V.sub.DDL within an internal circuit including the control circuit and the level shift circuits, is applied between the gate and source (the gate and drain) of each of the transistors of the output circuit. Therefore, if the breakdown voltage of the transistors of the output circuit has the same breakdown voltage as the transistors within the internal circuit, the transistors are so deteriorated that the reliability of the semiconductor device is reduced.
In order to enhance the reliability of the semiconductor device, a special manufacturing process different from a manufacturing process for the internal circuit is performed upon the output circuit. For example, the gate silicon oxide layer of the transistors of the output circuit is made thicker than that of the transistors within the internal circuit, which, however, increases the manufacturing cost.